An MS/MS analysis, or tandem analysis, is used with an ion trap mass spectrometer. In the MS/MS analysis, ions having a certain mass-to-charge ratio are selected from an analyzing sample as precursor ions. The selected precursor ions are dissociated with the Collision Induced Dissociation method, and the dissociated ions are mass-analyzed, whereby information on the mass and/or chemical structure of the object ions is obtained.
In normal ion trap mass spectrometers, an ion trap space is formed surrounded by a ring electrode and two end cap electrodes placed opposite each other with the ring electrode between them. The ring electrode has a hyperboloid-of-one-sheet-of-revolution internal surface, and the end cap electrodes have hyperboloid-of-two-sheets-of-revolution internal surfaces. When appropriate voltages are applied between the ring electrode and the end cap electrodes, a quadrupole electric field is generated in the ion trap space, and ions, whichever produced outside and brought inside or produced inside of the ion trap space, are contained, or trapped, in the ion trap space. When an MS/MS analysis is to be conducted, in order to select object precursor ions, the object ions are exclusively selected and left in the ion trap space, and the rest of the ions are discharged from there.
One of the ion selecting methods is as follows. When alternating current (AC) voltages of opposite phases having a certain frequency are applied to the two end cap electrodes, such ions whose secular frequency is the same as the frequency of the AC voltages vibrate resonantly. The amplitude of the resonant vibration increases gradually, and finally the ions escape from the ion trap space or collide with the surrounding electrodes. Thus non-object ions are eliminated and object ions are selectively left in the ion trap space. The mass-to-charge ratio of the ions that vibrate resonantly has a certain relationship with their secular frequency. When a wide band AC voltage having the frequency spectrum, as shown in FIG. 6, which has a notch at frequency f0(i.e., a wide band frequency devoid of the frequency f0) is applied to the end cap electrodes, ions having the mass-to-charge ratio m0 corresponding to the frequency f0 do not vibrate and remain in the ion trap space while other ions vibrate resonantly and are discharged from there. Thus the precursor ions are exclusively selected. Then a buffer gas is introduced to the ion trap space to promote dissociation of the precursor ions due to collisions with the buffer gas molecules. The dissociated ions, called productions, are then discharged from the ion trap space and analyzed.
The vibration frequency of ions in the ion trap space depends on some operating parameters of the ion trap, as well as the mass-to-charge ratio of the ions. For example, it depends on the amplitude of the primary RF voltage applied to the ring electrode. FIG. 7 shows an example of the relationship between the mass-to-charge ratio of an ion and the secular frequency (which corresponds to the notch frequency in an ion selection) of the ion, with the amplitude of the primary RF voltage as a variable parameter. Reference may be made as to the graph to the Publication No. 2000-323090 of Japanese Patent Application filed by the assignee of this application. The slope of the tangent of a curve in FIG. 7 represents the resolution of the mass analysis. When the mass-to-charge ratio is fixed, for example at 200 as shown in FIG. 7, the value of secular frequency, or the notch frequency, increases as the amplitude of the primary RF voltage is increased, and the slope of the tangent deceases as the notch frequency increases, which is apparent comparing the tangents P1 and P2. This means that an increase in the primary RF voltage leads to a higher resolution or an improved selectivity of the mass-to-charge ratio of ions. Thus it is preferable to choose such conditions in an ion selection that the vibrating frequency is higher (by increasing the amplitude of the primary RF voltage, for example) in order to select precursor ions at high resolution.
When precursor ions are selected under the condition that the vibrating frequency is very high with a large amplitude of the primary RF voltage, however, a portion of the precursor ions to be selected is also discharged and less ions remain in the ion trap space, which deteriorates the sensitivity of the overall analysis. That is, in the conventional methods as explained above, the resolution of the mass-to-charge ratio and the sensitivity of the analysis are in a trade-off, so that it was impossible to obtain both at high levels.